1. Field of the Invention
The present invention relates to a pixel circuit for active matrix display apparatus, particularly to a flat active matrix display controlled by full digital signal to reach the pixel circuit with the desired grey scale.
2. Description of the Prior Art
The flat display device has the advantages of light weight, low power consumption and low radiation etc. At present, the commercial flat display devices include the liquid crystal display and the organic light emitting diode display device, which have been widely used in the portable products, such as the notebook, digital camera, global positioning system and intellectual mobile phone, even the driving recorder etc.
The liquid crystal display is widely used particularly in the flat display device. Recently, the liquid crystal screen and the liquid crystal television are very popular, which are used to substitute the traditional bulky cathode ray tube (CRT) display and television. However, there are a lot of shortcomings for the liquid crystal display, for example: due to the limit for the characteristics of liquid crystal molecule, during the switching of image data, the liquid crystal molecules should be controlled and twisted to change the direction of their arrangement, the picture will be delayed, this delay will cause the residual image on the display. Thus, in order to respond the quick switch of multi-media image, the response speed of liquid crystal molecules should be improved.
When large-size flat display device is designed, in order to reach high resolution, shorter charging time is required. Due to the signal line will be lengthened with respect to larger size of flat display device, the inner resistance effect of signal line will be significant, which is called the RC-delay. The RC-delay will severely influence the uniformity for the luminance of flat display device, this is a main issue which should be resolved for large-size flat display device.
In order to solve the abovementioned issue of RC-delay, the conventional technique is to add a digital/analog signal switcher except the pixel circuit to control the charging time of capacitor. The common digital/analog signal switcher includes many driving circuits, which has to switch the digital signal into the analog signal, then inputs it into the pixel circuit for driving. The extra power will be increased in the process. Meantime, the driving force will be insufficient for large-size on high-resolution flat display device. Furthermore, the design and disposition of pixel will become much complicated. The complicate circuit will reduce the opening rate greatly, even more transistors and memories are required at the input end, which will increase the process difficulty and manufacturing cost greatly.
Therefore, in order to generate more efficient circuit device, it is necessary to research and develop the innovative pixel circuit technique, to improve the use efficiency, and reduce the manufacturing time and manufacturing cost.